Typeck existential types properly

src/librustc_typeck/check/mod.rs

@@ -1309,6 +1309,7 @@ pub fn check_item_type<'a,'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, it: &'tcx hir::Item
       hir::ItemKind::Union(..) => {
         check_union(tcx, it.id, it.span);
       }
+      hir::ItemKind::Existential(..) |
       hir::ItemKind::Ty(..) => {
         let def_id = tcx.hir.local_def_id(it.id);
         let pty_ty = tcx.type_of(def_id);

src/test/ui/impl-trait/auto-trait-leak.rs

@@ -23,7 +23,9 @@ fn main() {
 // return type, which can't depend on the obligation.
 fn cycle1() -> impl Clone {
     //~^ ERROR cycle detected
+    //~| ERROR cycle detected
     send(cycle2().clone());
+    //~^ ERROR cannot be sent between threads safely
 
     Rc::new(Cell::new(5))
 }

src/test/ui/impl-trait/auto-trait-leak.stderr

@@ -1,29 +1,67 @@
-error[E0391]: cycle detected when processing `cycle1`
-  --> $DIR/auto-trait-leak.rs:24:1
-   |
-LL | fn cycle1() -> impl Clone {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-note: ...which requires evaluating trait selection obligation `impl std::clone::Clone: std::marker::Send`...
-note: ...which requires processing `cycle2::{{impl-Trait}}`...
-  --> $DIR/auto-trait-leak.rs:31:16
-   |
-LL | fn cycle2() -> impl Clone {
-   |                ^^^^^^^^^^
-note: ...which requires processing `cycle2`...
-  --> $DIR/auto-trait-leak.rs:31:1
-   |
-LL | fn cycle2() -> impl Clone {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^
-note: ...which requires evaluating trait selection obligation `impl std::clone::Clone: std::marker::Send`...
-note: ...which requires processing `cycle1::{{impl-Trait}}`...
+error[E0391]: cycle detected when processing `cycle1::{{impl-Trait}}`
   --> $DIR/auto-trait-leak.rs:24:16
    |
 LL | fn cycle1() -> impl Clone {
    |                ^^^^^^^^^^
-   = note: ...which again requires processing `cycle1`, completing the cycle
-note: cycle used when type-checking all item bodies
+   |
+note: ...which requires processing `cycle1`...
+  --> $DIR/auto-trait-leak.rs:24:1
+   |
+LL | fn cycle1() -> impl Clone {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires evaluating trait selection obligation `impl std::clone::Clone: std::marker::Send`...
+note: ...which requires processing `cycle2::{{impl-Trait}}`...
+  --> $DIR/auto-trait-leak.rs:33:16
+   |
+LL | fn cycle2() -> impl Clone {
+   |                ^^^^^^^^^^
+note: ...which requires processing `cycle2`...
+  --> $DIR/auto-trait-leak.rs:33:1
+   |
+LL | fn cycle2() -> impl Clone {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires evaluating trait selection obligation `impl std::clone::Clone: std::marker::Send`...
+   = note: ...which again requires processing `cycle1::{{impl-Trait}}`, completing the cycle
 
-error: aborting due to previous error
+error[E0391]: cycle detected when processing `cycle1::{{impl-Trait}}`
+  --> $DIR/auto-trait-leak.rs:24:16
+   |
+LL | fn cycle1() -> impl Clone {
+   |                ^^^^^^^^^^
+   |
+note: ...which requires processing `cycle1`...
+  --> $DIR/auto-trait-leak.rs:24:1
+   |
+LL | fn cycle1() -> impl Clone {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires evaluating trait selection obligation `impl std::clone::Clone: std::marker::Send`...
+note: ...which requires processing `cycle2::{{impl-Trait}}`...
+  --> $DIR/auto-trait-leak.rs:33:16
+   |
+LL | fn cycle2() -> impl Clone {
+   |                ^^^^^^^^^^
+note: ...which requires processing `cycle2`...
+  --> $DIR/auto-trait-leak.rs:33:1
+   |
+LL | fn cycle2() -> impl Clone {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: ...which again requires processing `cycle1::{{impl-Trait}}`, completing the cycle
 
-For more information about this error, try `rustc --explain E0391`.
+error[E0277]: `std::rc::Rc<std::string::String>` cannot be sent between threads safely
+  --> $DIR/auto-trait-leak.rs:27:5
+   |
+LL |     send(cycle2().clone());
+   |     ^^^^ `std::rc::Rc<std::string::String>` cannot be sent between threads safely
+   |
+   = help: within `impl std::clone::Clone`, the trait `std::marker::Send` is not implemented for `std::rc::Rc<std::string::String>`
+   = note: required because it appears within the type `impl std::clone::Clone`
+note: required by `send`
+  --> $DIR/auto-trait-leak.rs:16:1
+   |
+LL | fn send<T: Send>(_: T) {}
+   | ^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+
+Some errors occurred: E0277, E0391.
+For more information about an error, try `rustc --explain E0277`.